The CV391 was designed for transmitter use in the HF and low VHF parts of the spectrum. The design was an 807 in an all glass format. However, the specification quotes the prototype as the LS650.
In addition to RF power, the valves were designated for service as audio power amplifiers. Here the valve was to operate in class AB1 and a pair would deliver 15 Watts at 3% total distortion.
The specification gives the base as B8G but Loctal or B8B is the usual name. This exhibit was made by the American ITT company, but as it says Made in England it was in fact made by STC.
The anode with side flanges to maximise heat dissipation from the carbonised metal.
The envelope also carries the commercial designation of 5B/255M. STC also offered the re-designed 807 as the 5B/254M, complete with anode top cap.
The wide glass tube envelope is 28 mm in diameter and, excluding the B8B base pins, is 76 mm tall.
References: 1040 & data-sheet Type CV391 was first introduced in 1957. See also1957 adverts.